Low-power lamps, such as light-emitting diode (“LED”) lamps, may be powered by a low-voltage power supply (e.g., 12 V). A transformer may be used to adapt a high-voltage mains supply (e.g., 120 V) for use with a low-voltage LED lamp. One type of transformer circuit, a self-oscillating transformer, uses a transistor-based bridge to rectify the incoming mains voltage into a half-sine wave, thereby doubling the frequency of the incoming voltage before it is applied to the transformer. The circuit is self-oscillating in the sense that the bridge transistors are controlled by bias voltages created by auxiliary windings of the transformer.
Self-oscillating transformers include startup circuits to initiate oscillation at the beginning of each AC cycle. LED lamps, however, draw such a small amount of power that the transformer startup circuit may fail to cause enough current flow in the transformer to initiate an oscillation. Instead, the bias voltages created by auxiliary windings of the transformer fall toward zero, thereby shutting off control to the bridge transistors. The transformer may get “stuck” in this off position, creating an undesirable interruption in the power to the low-voltage lamp. A need therefore exists for a system that initiates and maintains the self-oscillation of the transformer at start-up.